1. Field of the Invention
The present invention relates to a device for purifying exhaust gas for an engine.
2. Related Art
An exhaust gas purification apparatus for an engine which is operated with a lean air-fuel ratio is known. The apparatus comprises a SOx storage, such as a NOx catalyst, arranged in an exhaust passage of the engine, for temporarily storing SOx in the inflowing exhaust gas, a bypass passage connecting the exhaust passages upstream and downstream of the NOx catalyst to each other while bypassing the NOx catalyst, a switching valve capable of being turned between a position where substantially all the exhaust gas is introduced into the NOx catalyst and a bypass position where a small part of the exhaust gas is introduced into the NOx catalyst while leading the remaining exhaust gas into the bypass passage wherein, when the SOx stored in the NOx catalyst should be discharged therefrom, the switching valve is held at the bypass position while the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is turned to the stoichiometric or rich air-fuel ratio. When the switching valve is held at the bypass position, the amount of the exhaust gas flowing into the NOx catalyst is reduced, and therefore, the amount of the reducing agent required to make the air-fuel ratio of the exhaust gas flowing into the NOx catalyst the stoichiometric or rich air-fuel ratio can be reduced. Once the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is turned to the stoichiometric or rich air-fuel ratio, SOx stored in the NOx catalyst is discharged in the form of SO2, H2S from the NOx catalyst and the amount of SOx stored in the NOx catalyst is thus reduced.
However, when the switching valve is held at the bypass position, a large part of the exhaust gas circumvents the NOx catalyst, with the result that a large amount of HC and CO may be discharged into the atmosphere.
Although the air-fuel ratio of the exhaust gas flowing into the NOx catalyst is the stoichiometric or rich air-fuel ratio, the mean air-fuel ratio of the exhaust gas flowing through the exhaust passage downstream of an outlet of the bypass passage is lean. Therefore, it may be considered that an arrangement of an auxiliary catalyst having an oxidation ability in the exhaust passage downstream of the outlet of the bypass passage could solve the above-mentioned problem.
However, if the amount of the reducing agent in the exhaust gas flowing into the auxiliary catalyst is small when the temperature of the auxiliary catalyst is high, another problem may arise that the SO2 and H2S discharged from the NOx catalyst are oxidized into the sulfate SO3 at the auxiliary catalyst and the SO3 is discharged into the atmosphere.